A root is a plant organ that has adapted to acquire water and nutrients from the environment (Schiefelbein J. W. et al., Plant Cell, 9:1089–1098, 1997). A root consists of epidermis, cortex and vascular tissues in radial rows or concentric circles (Esau K., Anatomy of Seed Plants, Ed. 2, 215–245, 1997; Dolan L. et al., Development, 119:71–84, 1993; Raven P. H. et al., Biology of Plants, 6th Ed. Worth Publishers, New York, 1999). In the longitudinal section, a root can be divided into three different regions; cell division, elongation and maturation region (Dolan L. et al., Development, 119:71–84, 1993; Baluska F. et al., Plant Physiol., 112:3–4, 1996). The region of cell division carries out new cell divisions. The cells derived from the region of cell division expand and elongate mostly in the region of elongation. The elongated cells begin to differentiate in a region of maturation, where root hairs and secondary roots are initiated. The cell elongation and maturation in a root are controlled by the extensibility of cell wall and the turgor pressure inside the cell (Cosgrove D. J., BioEssays, 18533–540, 1996).
It is known that the extent of plant cell elongation is confined by cell walls. The cell wall is composed of polysaccharides, proteins, phenolic compounds and other materials (Varner J. E. et al., Cell, 56:231–239, 1989). The plant cell wall plays a determinative role in establishing the size and shape of a plant cell. For elongation or maturation, however, a plant cell needs to selectively modify its cell wall. The agents for cell wall modification in the plant cell include various cell wall components, such as expansins, endoglucanases, xyloglucan endotransglycosylases and hydroxyl radicals (Cosgrove D. J., Annu. Rev. Plant Physiol. Plant Mol. Biol., 50:391–417, 1999; Cosgrove D. J., Plant Physiol. Biochem., 38:109–124, 2000a; Cosgrove D. J., Nature, 407:321–326, 2000b).
Expansins are generally considered as primary agents for cell wall elongation (Vissenberg K. et al., Plant Cell, 12:1229–1237, 2000). Expansins cause wall creep by loosening hydrogen bonds between cellulose microfibrils and matrix polymer (McQeen-Mason S. et al., Proc. Natl. Acad. Sci. USA, 91:6574–6578, 1994; Cosgrove D. J., Plant Cell, 9:1031–1041, 1997). Since the first cloning of an expansin gene (Shcherban T. Y. et al., Proc. Natl. Acad. Sci. USA, 92:9245–9249, 1995), many expansin genes have been identified from a variety of plant species. And they are known to form a multigene family (Cosgrove D. J., Plant Physiol., 118:333–339, 1998). The expansin genes are classified into three subfamilies, α-, β- and γ-expansin subfamilies, based on their phylogenetic relationship (Li Y. et al., Plant Physiol., 128:854–864, 2002). The (α-expansins compose a major portion of the expansins, including the ones from tomato (Lycopersicon esculentum) (Keller E. et al., Plant J., 8:795–802, 1995), rice (Oryza sativa) (Cho H-T. et al., Plant Physiol., 113:1137–1143, 1997a), oat (Avena sativa) (Li Z-C et al., Planta, 191:349–356, 1993) and Arabidopsis(Arabidopsis thaliana) (Cosgrove D. J., Plant Physiol., 118: 333–339, 1998; Li Y. et al., Plant Physiol., 128:854–864, 2002). The α-expansin subfamily can be further divided into A, B, C and D groups (Link B. M. et al., Plant Physiol, 118:907–916, 1998).
Expression patterns of the α-expansin genes have been extensively studied in deepwater rice and tomato. It was reported that the transcript of an expansin gene in deepwater rice, OsEXP4, increases in abundance before onset of cell wall extensibility and faster growth, supporting the role of expansins in cell elongation (Cho H-T et al., Plant J, 15:805–812, 1998). Also, in tomato, expression of the LeEXP18 gene was localized in a group of cells in the shoot apical meristem where incipient leaf primordium initiation takes place (Reinhardt D. et al., Plant Cell, 10:1427–1437, 1998). International Patent Publication No. WO02086066 discloses a novel β-expansin polypeptide for modifying the structure of cell walls in a plant and a nucleotide sequence encoding the same. U.S. Pat. No. 5,929,303 discloses a fruit-specific and ripening regulation expansin gene.
Studies of a variety of expansin genes and their tissue-specific expression patterns show that different expansin genes may play different roles in various cell types during organ development in plants (Rose et al., Proc. Natl. Acad. Sci, USA, 94:5955–5950, 1997). Therefore, it is still necessary to study new expansin genes and their functions.